Use of thiouronium salts as brighteners for aqueous acidic electronickelization baths

ABSTRACT

Nickel-plated shaped parts are produced by galvanic precipitation of nickel from aqueous-acid baths which contain as essential constituents one or several nickel salts, one or several inorganic acids and one or several brighteners. As brighteners are used thiourea salts having general formula (I), in which R 1  to R 4  stand for hydrogen, C 1  to C 18  -alkyl, which may be substituted by carboxyl groups, C 1  to C 4  -alkoxycarbonyl groups or cyano groups, C 2  to C 12  -alkenyl, C 2  to C 12  -alkinyl, C 5  to C 8  -cycloalkyl, C 7  to C 12  -phenylalkyl or phenyl, which may be substituted by one or two C 1  to C 4  -alkyl residues, C 1  to C 4  -alkoxy residues, halogen atoms, hydroxyl groups, phenyl groups, phenyl residues or C 1  to C 4  alkoxycarbonyl groups; Y sands for a chemical bond or for linear or branched alkylene, alkenylene or alkinylene having each up to 20 C. atoms; A stands for hydrogen or a group having the formulae: --CO--H, --CO--R 5 , --CO--OH, --CO--OR 5 , --CO--NR 6  R 7 , --CO--CH 2  --CO--OR 5 , --O--CO--H, --O--CO--R 5 , --NR 6  --CO--R 5 , --NR 6  --CO--R 5 , --OR 5 , --SO 2  --R 5 , --SO 2  --OH, --SO 2  --OR 5 , --PO(PH) 2 , --PO(OH)(OR 5 ), --PO(OR 5 ) 2 , OPO(OH) 2 , --OPO(PH)(PR 5 ) or --OPO(OR 5 ) 2  in which R 5  stands for C 1  to C 12  -alkyl, C 2  to C 12  -alkenyl, C 2  to C 12  -alkinyl, C 5  to C 8  -cycloalkyl, C 7  to C 12  -phenylalkyl or phenyl, which may be substituted by one or two C 1  to C 4  -alkyl residues, halogen atoms, hydroxyl groups, phenyl residues or C 1  to C 4  -alkoxycarbonyl groups; and R 6  and R 7  stand for hydrogen or C 1  to C 4  -alkyl; n is a number from 1 to 4 and X.sub.(--) stands for s water solubility-promoting, n-valent inorganic or organic anion.

The present invention relates to a process for producing nickelizedshaped articles by electrodeposition of nickel from aqueous acidic bathsusing thiouronium salts as brighteners. The invention also relates tothose thiouronium salts which are novel compounds.

It is also known that acidic nickel electrolytes must contain smallamounts of organic substances if the electronickelization is to producea bright, ductile and level deposition of the metal. Such brighteners,which in general are divided into primary and secondary brighteners, arecustomarily used in the form of combinations comprising a plurality ofthese agents in order to enhance the effect.

Praktische Galvanotechnik, Eugen G. Lenze Verlag, Saulgau, 4th edition1984, pages 268 to 271 (1) describes customary brighteners for nickelelectrolytes. Although the compounds are classified as primary orsecondary brighteners or leveling agents, it is admitted at the sametime that clear-cut classification is not always possible. Thebrightening compounds mentioned are:

sulfonimides, e.g. benzoic sulfimide

sulfonamides

benzenesulfonic acids, e.g., mono-, di- and tribenzenesulfonic acid

naphthalenesulfonic acids, e.g., mono-, di- and trinaphthalenesulfonicacid

alkylsulfonic acids

sulfinic acid

arylsulfone sulfonates

aliphatic compounds with ethylene and/or acetylene bonds, e.g.,butynediol

single- and multi-ring nitrogen-containing heterocycles with or withoutfurther hetero atoms such as sulfur or selenium

coumarin

amines and quaternary ammonium compounds as leveling agents

saccharin.

DE-B-1 191 652 (2) describes single- or multi-ring heterocyclic nitrogenbases of the aromatic type in quaternized form such as pyridinium salts,e.g. 2-(1-pyridinio)ethanesulfate, as levelers, ie. brighteners, foracidic nickel-plating baths. These agents are used together withcustomary basic brighteners such as benzene-m-disulfonic acid,diaryldisulfimides or sulfonamides.

U.S. Pat. No. 3,630,857 (3) discloses using nitrile group-containingthiourea derivatives of the general formula ##STR1## where the Rs areindependently of one another hydrogen, alkyl, alkenyl or alkynyl each offrom 1 to 4 carbon atoms and n is from 1 to 4, as brighteners foraqueous acidic electronickelization baths.

In commercial practice, it is customary to combine alkenylsulfonic acidssuch as sodium vinylsulfonate or sodium allylsulfonate with otherbrighteners such as propargyl alcohol, 2-butyne-1,4-diol,propynesulfonic acid or 3-(1-pyridinio)-1-propanesulfonate.

However, the prior art agents generally need to be used in relativelyhigh concentrations in the nickel electrolyte baths used.

It is an object of the present invention to provide an improved processfor producing nickelized shaped articles using brighteners that aresuperior or at least equal in brightening to, for example,2-(1-pyridinio)ethanesulfate or 3-(1-pyridinio)-1-propanesulfonate, butcan be used in a lower concentration.

We have found that this object is achieved by a process for producingnickelized shaped articles by electrodeposition of nickel from aqueousacid baths containing as essential constituents one or more nickelsalts, one or more inorganic acids and one or more brighteners, whichcomprises using as brighteners thiouronium salts of the general formulaI ##STR2## where R¹ to R⁴ are each hydrogen, C₁ -C₁₈ -alkyl, which maybe carboxyl-, C₁ -C₄ -alkoxycarbonyl- or cyano-substituted, C₂ -C₁₂-alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈ -cycloalkyl, C₇ -C₁₂ -phenylalkyl orphenyl which may be substituted by one or two substituents selected fromthe group consisting of C₁ -C₄ -alkyl, C₁ -C₄ -alkoxy, halogen,hydroxyl, phenyl and C₁ -C₄ -alkoxycarbonyl,

Y is a chemical bond or linear or branched alkylene, alkenylene oralkynylene having in each case up to 20 carbon atoms,

A is hydrogen or a group of the formula --CO--H, --CO--R⁵, --CO--OH,--CO--OR⁵, --CO--NR⁶ R⁷, --CO--CH₂ --CO--OR⁵, --O--CO--H, --O--CO--R⁵,--NR⁶ --CO--R⁵, --OR⁵, --SO₂ --R⁵, --SO₂ --OH, --SO₂ --OR⁵, --PO(OH)₂,--PO(OH)(OR⁵), --PO(OR⁵)₂, OPO(OH)₂, --OPO(OH)(OR⁵) or --OPO(OR⁵)₂,where

R⁵ is C₁ -C₁₂ -alkyl, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈-cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted byone or two substituents selected from the group consisting of C₁ -C₄-alkyl, halogen, hydroxyl, phenyl and C₁ -C₄ -alkoxycarbonyl, and

R⁶ and R⁷ are each hydrogen or C₁ -C₄ -alkyl,

n is from 1 to 4, and

X.sup.⊖ is an n-valent inorganic or organic anion that promotessolubility in water.

Suitable C₁ -C₄ -alkyl for R⁶ to R¹⁰, R¹² and R¹³ and for substituentson phenyl are n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl,tert-butyl and in particular methyl and ethyl.

Examples of straight-chain or branched C₁ -C₁₈ - or C₁ -C₁₂ -alkyl forR¹ to R⁴, R⁵ and (R¹¹) in addition to the abovementioned C₁ -C₄ -alkylradicals are n-amyl, isoamyl, sec-amyl, tert-amyl, neopentyl, n-hexyl,n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, isononyl, n-decyl, n-undecyl,n-dodecyl, n-tetradecyl, n-hexadecyl and n-octadecyl. Of these, C₁ -C₄-alkyl is preferred.

Examples of suitable carboxyl-, C₁ -C₄ -alkoxycarbonyl- orcyano-substituted C₁ -C₁₈ -alkyl for R¹ to R⁴ are 2-carboxyethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl and 2-cyanoethyl.

Suitable C₅ -C₈ -cycloalkyl for R¹ to R⁴, R⁵ and R¹¹ is in particularcyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, methylcyclopentyl,dimethylcyclopentyl, methylcyclohexyl, dimethylcyclohexyl orethylcyclohexyl. Of these, preference is given to cyclopentyl andcyclohexyl.

Examples of suitable C₇ -C₁₂ -phenylalkyl for R¹ to R⁴, R⁵ and R¹¹ are1-phenylethyl, 2-phenylethyl, 1-phenylpropyl, 2-phenylpropyl,3-phenylpropyl, 2-phenylprop-2-yl, 4-phenylbutyl,2,2-dimethyl-2-phenylethyl, 5-phenylamyl, 6-phenylhexyl and inparticular benzyl.

In monosubstituted phenyl for R¹ to R⁴, R⁵ and R¹¹ the substitutionpattern is ortho, meta or preferably para and in disubstituted phenylthe substituents are preferably in the 2,4-position, for example as in2,4-xylyl. If substituents are present, the degree of substitution ispreferably 1. But particular preference is given to unsubstitutedphenyl.

Suitable C₁ -C₄ -alkoxy radicals are in particular methoxy and ethoxybut also n-propoxy, n-butoxy, isobutoxy, sec-butoxy and tert-butoxy.

Examples of C₁ -C₄ -alkoxycarbonyl are n-propoxycarbonyl,isopropoxycarbonyl, n-butoxycarbonyl, isobutoxycarbonyl,sec-butoxycarbonyl, tert-butoxycarbonyl but in particular ethoxycarbonyland methoxycarbonyl.

The term halogen atom herein encompasses fluorine, iodine and inparticular bromine and especially chlorine.

Examples of straight-chain or branched C₂ -C₁₂ -alkylene for R¹ to R⁴,(R⁵ and R¹¹) are vinyl, allyl, methallyl, 3-butenyl, 4-pentenyl,5-hexenyl, 7-octenyl, 9-decenyl, 11-dodecenyl, citronellolyl, geraniolyland linaloolyl.

Examples of suitable straight-chain or branched C₂ -C₁₂ -alkynyl for R¹to R⁴, R⁵ and R¹¹ are ethynyl and 2-propynyl.

R⁶ to R¹⁰, R¹² and R¹³ are each preferably hydrogen, methyl or ethyl.

m is preferably from 0 to 8, in particular from 0 to 5, especially from0 to 3.

Suitable n-valent anions X are the customary, normallywater-solubilizing inorganic or organic anions, in particular chloride,bromide, fluoride, sulfate, hydrogensulfate, methanesulfonate,trifluoromethanesulfonate, 2-hydroxyethanesulfonate, p-toluenesulfonate,nitrate, tetrafluoroborate, perchlorate,1-hydroxyethane-1,1-diphosphonate, dihydrogenphosphate, hydrogenphosphate, phosphate, formate, acetate, oxalate, citrate and tartrate.

Of these, anions with one or two charges (n=1 or 2) are preferred, inparticular fluoride, sulfate, methanesulfonate, nitrate andtetrafluoroborate but especially chloride and bromide.

If A is a carboxylic acid, sulfonic acid, phosphonic acid or phosphoricacid function, the thiouronium salts I can also be present as betaineswhich can be formed by elimination of HX from the thiouronium salts I.

A preferred embodiment comprises using thiouronium salts I wherein R¹ toR⁴ are each hydrogen, C₁ -C₄ -alkyl, allyl or phenyl which may besubstituted by one or two C₁ -C₄ -alkyl radicals. In particular, R¹ toR⁴ are all hydrogen or three hydrogens and one allyl.

In a further preferred embodiment, the thiouronium salts used have thegeneral formula Ia ##STR3## where R¹ to R⁴, n and X.sup.⊖ are each asdefined above, R⁸ to R¹⁰ are each hydrogen or C₁ -C₄ -alkyl, R¹¹ ishydrogen, C₁ -C₁₂ -alkyl, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈-cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted byone or two C₁ -C₄ -alkyl radicals, and m is from 0 to 10.

In a further preferred embodiment, the thiouronium salts have thegeneral formula Ib ##STR4## where R¹ to R⁴, R⁸ to R¹¹, m, n and X.sup.⊖are each as defined above.

In a further preferred embodiment, the thiouronium salts have thegeneral formula Ic ##STR5## where R¹ to R⁴, R⁸, R⁹, R¹¹, n and X.sup.⊖are each as defined above and Z is a group of the formula --CR¹² ═CR¹³-- or --C.tbd.C-- where R¹² and R¹³ are each hydrogen or C₁ -C₄ -alkyl.

In a further preferred embodiment, the thiouronium salts have thegeneral formula Id ##STR6## where R¹ to R⁴, R⁸ to R¹¹, m, n and X.sup.⊖are each as defined above.

In a further preferred embodiment, the thiouronium salts have thegeneral formula Ie ##STR7## where R¹ to R⁴, R⁶ to R⁹, m, n and X.sup.⊖are each as defined above.

In a further preferred embodiment, the thiouronium salts have thegeneral formula If ##STR8## where R¹ to R⁴, R⁸ to R¹¹, m, n and X.sup.⊖are each as defined above.

In a further defined embodiment, the thiouronium slats have the generalformula Ig ##STR9## where R¹ to R⁴, R⁸, R⁹, Z, m, n and X.sup.⊖ are eachas defined above.

Since the thiouronium salts I act in a way that is typical of secondarybrighteners, they are preferably used combined with at least onefurther, normally primary brightener or else, as the case may be, withone or more further secondary brighteners. Examples of suitable primarybrighteners are sodium vinylsulfonate and sodium allylsulfonate, andexamples of suitable secondary brighteners are 2-butyne-1,4-diol orpropargyl alcohol.

The aqueous acidic nickel electrolyte baths used contain one or usuallymore than one nickel salt, for example nickel sulfate and nickelchloride, one or more inorganic acids, preferably boric acid andsulfuric acid, as brighteners the compounds I alone or preferablycombined with further, customary brighteners and optionally furthercustomary auxiliaries and additives in the concentrations customarytherefor, for example wetting agents and pore inhibitors. Customaryaqueous acidic nickel electrolytes ("Watts electrolytes") have thefollowing basic composition:

200-300 g/l of NiSO₄.7 H₂ O

30-150 g/l of NiCl₂.6 H₂ O

30-50 g/l of H₃ BO₃.

The electrolyte bath pH is customarily within the range from 3 to 6,preferably within the range from 4 to 5. This pH is conveniently setwith a strong mineral acid, preferably sulfuric acid.

The compounds I are present in the electrolyte baths in lowconcentrations, in general within the range from 0.01 to 0.5 g/l,preferably within the range from 0.025 to 0.3 g/l. The concentrations offurther, customary brighteners are in each case normally within therange from 0.1 to 10 g/l, in particular from 0.1 to 2.0 g/l.

The nickel electrolyte baths described can be used to electroplate inparticular nickel coatings onto shaped articles made of steel, but alsoonto shaped articles made of other materials, for example brass, whichhave been pretreated as usual. The electroplating is in general carriedout at from 30° to 80° C., preferably at from 40° to 60° C.

The compounds I of the invention are notable for extremely powerfulbrightening. In general they produce better brightness than thecustomary brighteners and, what is more, usually at a distinctly lowerconcentration in the nickel electrolyte baths.

The thiouronium salts I described are advantageously preparable byreacting the corresponding precursor of the general formula II

    Nuc-Y-a                                                    (II)

where Nuc is a nucleofugic leaving group, preferably chlorine orbromine, with a thiourea of the general formula III ##STR10## and, ifdesired, subsequently exchanging the anion Nuc.sup.⊖ for X.sup.⊖.

Suitable thioureas III include for example unsubstituted thiourea,N-methylthiourea, N,N'-dimethylthiourea, N,N,N',N'-tetramethylthiourea,N-ethylthiourea, N,N'-diethylthiourea, N,N,N',N'-tetraethylthiourea,N-phenylthiourea, N,N'-diphenylthiourea, N-phenyl-N-methylthiourea,N-phenyl, N'-methylthiourea, N,N'-dibutylthiourea, N-benzylthiourea,N-allylthiourea or N,N'-dicyclohexylthiourea.

The reaction of components II and III is advantageously carried out inan inert organic solvent such as toluene, xylene, petroleum ether,naphtha, cyclohexane, acetone, tetrahydrofuran, dioxane, methanol,ethanol, isopropanol, ethyl acetate or methyl benzoate or in a mixturethereof. However, the reaction can also be carried out in water or in asingle-phase or two-phase mixture of water and one or more organicsolvents, preferably polar organic solvents. In the case of two-phasemixtures it is possible to use a customary phase transfer catalyst. Thereaction is generally carried out at from 40° to 130° C., in particularat from 60° to 110° C., and at atmospheric pressure.

Since some of the thiouronium salts I are novel substances, the presentinvention also relates to these novel compounds.

The present invention accordingly provides thiouronium salts of thegeneral formula Ih ##STR11## where R¹ to R⁴, R⁸ to R¹⁰, m, n and X.sup.⊖are each as defined above and R¹⁴ is C₇ -C₁₂ -phenylalkyl or phenylwhich may be substituted by one or two C₁ -C₄ -alkyl radicals.

The present invention further provides thiouronium salts of the generalformula Ij ##STR12## where R¹ to R⁴, R⁸, R⁹, R¹¹, n and X.sup.⊖ are eachas defined above, subject to the proviso that at least one of R⁸ and R⁹is not hydrogen.

The present invention further provides thiouronium salts of the generalformula Ib ##STR13## where R¹ to R⁴, R⁸ to R¹¹, m, n and X.sup.⊖ areeach as defined above.

The present invention further provides thiouronium salts of the generalformula Ik ##STR14## where R¹ to R⁴, n and X.sup.⊖ are each as definedabove and R¹⁵ is the group ##STR15## where R⁶ to R¹¹, Z and m are eachas defined above, subject to the proviso that at least one of R¹ to R⁴is allyl.

The thiouronium salts Ih and Ij are subgroups of the compounds Ia andthe thiouronium salts Ik are subgroups of the compounds Ic, Ie and If.At the same time, the use of the thiouronium salts Ih, Ij and Ik in thenovel process for producing nickelized shaped articles is in each case afurther preferred embodiment.

Preparation Examples EXAMPLE 1 Preparation of2-(benzyloxycarbonylmethyl)thiouronium chloride

7.6 g (0.1 mol) of thiourea were dissolved in 50 ml of acetone. 18.5 g(0.1 mol) of benzyl chloroacetate were added dropwise and the mixturewas heated at the boil for two hours. After cooling down to roomtemperature, the precipitated product was filtered off and washed withdiethyl ether. Drying left 23.5 g (corresponding to a yield of 90%) ofthe title compound in the form of colorless crystals. The product wasmore than 99% pure.

EXAMPLE 2 Preparation of 2-(1-carboxypropyl)thiouronium bromide

Example 1 was repeated with thiourea and 2-bromobutyric acid, affordingthe title compound in a yield of 79%.

EXAMPLE 3 Preparation of 2-(2-carboxy-2-propyl)thiouronium chloride

Example 1 was repeated with thiourea and 2-chloro-2-methylpropionicacid, affording the title compound in a yield of 74%.

EXAMPLE 4 Preparation of 2-(N,N-dimethylcarbamoylmethyl)thiouroniumchloride

Example 1 was repeated with thiourea and 2-chloro-N,N-dimethylacetamide,affording the title compound ina yield of 89%.

EXAMPLE 5 Preparation of 2-(4-oxopentyl)thiouronium chloride

Example 1 was repeated with thiourea and 3-chloropropyl methyl ketone,affording the title compound in a yield of 71%.

EXAMPLE 6 Preparation ofN-allyl-S-(N,N-dimethylcarbamoylmethyl)thiouronium chloride

Example 1 was repeated with N-allylthiourea and2-chloro-N,N-dimethylacetamide, affording the title compound in a yieldof 79%.

EXAMPLE 7 Preparation of N-allyl-S-(2-methoxyethyl)thiouronium bromide

Example 1 was repeated with -allylthiourea and 2-bromoethyl methylether, affording the title compound in a yield of 69%.

EXAMPLE 8 Preparation of N-allyl-S-(3-carboxy-2-butenyl)thiouroniumbromide

Example 1 was repeated with N-allylthiourea and 4-bromo-2-methylcrotonicacid, affording the title compound in a yield of 70%.

Use Examples

The products prepared in Examples 1 to 8 were used as brighteners inweakly acidic electroplating baths for the deposition of nickel.

The aqueous nickel electrolyte used had the following composition:

300 g/l of NiSO₄.7 H₂ O

60 g/l of NiCl₂.6 H₂ O

45 /l of H₃ BO₃

2 g/l of saccharin

0.8 g/l of vinylsulfonic acid, sodium salt

x g/l of brightener as per table

0.5 g/l of a fatty alcohol derivative of the formula C₁₂ H₂₅ /C₁₄ H₂₉--O--(CH₂ CH₂ O)₂ --SO₃ Na as wetting agent.

The pH of the electrolyte was set to 4.2 with sulfuric acid.

Brass panels were used. Prior to being coated with nickel they had beencathodically degreased in a conventional manner in an alkalineelectrolyte. They were nickelized in a 250 ml Hull cell at 55° C. with acurrent strength of 2A over a period of 10 minutes. The panels were thenrinsed with water and dried over compressed air.

The table below shows the results of these experiments. It can be seenthat the brighteners of the invention produced better brightness thanthe prior art brighteners, in some instances at a distinctly lowerconcentration in the nickel electrolyte bath.

                  TABLE                                                           ______________________________________                                        Test results of electronickelization                                                 Brightener  Concentration                                                                             Brightness                                     ______________________________________                                        Example No.                                                                    9       of Example No. 1                                                                            0.1         5                                          10       of Example No. 2                                                                            0.2         5                                          11       of Example No. 3                                                                            0.2         5                                          12       of Example No. 4                                                                            0.2         5                                          13       of Example No. 5                                                                            0.2         5                                          14       of Example No. 6                                                                            0.2         5                                          15       of Example No. 7                                                                            0.2         5                                          16       of Example No. 8                                                                            0.3         5                                          for                                                                           comparison                                                                    A        2-(1-pyridinio)-                                                                            0.3         4-5                                                 ethanesulfate                                                        B        3-(1-pyridinio)-                                                                            0.3         4-5                                                 propanesulfonate                                                     ______________________________________                                         Rating scheme for brightness:                                                 5 = excellent (perfect specular gloss)                                        4 = good (almost specular gloss)                                              3 = moderate                                                                  2 = poor                                                                      1 = no brightness                                                             Comparative compound A is known from (2).                                     Concentration: [g/l                                                      

We claim:
 1. A process for producing nickelized shaped articles by electrodeposition of nickel from an aqueous acidic bath containing one or more nickel salts, one or more inorganic acids and one or more brighteners, which comprises using as brighteners thiouronium salts of formula I ##STR16## where R¹ to R⁴ are each hydrogen, C₁ -C₁₈ -alkyl, which may be carboxyl-, C₁ -C₄ -alkoxycarbonyl- or cyano-substituted, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈ -cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted by one or two substituents selected from the group consisting of C₁ -C₄ -alkyl, C₁ -C₄ -alkoxy, halogen, hydroxyl, phenyl and C₁ -C₄ -alkoxycarbonyl,Y is a chemical bond or linear or branched alkylene, alkenylene or alkynylene having in each case 1 to 20 carbon atoms, A is hydrogen or a group of the formula --CO--H, --CO--R⁵, --CO--OH, --CO--OR⁵, --CO--NR⁶ R⁷, --CO--CH₂ --CO--OR⁵, --O--CO--H, --O--CO--R⁵, --NR⁶ --CO--R⁵, --OR⁵, --SO₂ --R⁵, --SO₂ --OH, --SO₂ --OR⁵, --PO(OH)(OR⁵), or --PO(OR⁵)₂, where R⁵ is C₁ -C₁₂ -alkyl, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈ -cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted by one or two substituents selected from the group consisting of C₁ -C₄ -alkyl, halogen, hydroxyl, phenyl and C₁ -C₄ -alkoxycarbonyl, and R⁶ and R⁷ are each hydrogen or C₁ -C₄ -alkyl, n is from 1 to 4, and X.sup.⊖ is an n-valent inorganic or organic anion that promotes solubility in water.
 2. The process of claim 1, wherein A is selected from the group consisting of --CO--H, --CO--R⁵, --CO--OH, --CO--OR⁵, --CO--NR⁶ R⁷, --CO--CH₂ --CO--OR⁵, --O--CO--H, --O--CO--R⁵, --NR⁶ --CO--R⁵, and --OR⁵.
 3. The process of claim 1 wherein R¹ to R⁴ are each hydrogen, C₁ -C₄ -alkyl, allyl or phenyl which may be substituted by one or two C₁ -C₄ -alkyl radicals.
 4. The process of claim 1, wherein said thiouronium salt has formula Ia ##STR17## where R¹ to R⁴, n and X.sup.⊖ are each as defined above, R⁸ to R¹⁰ are each hydrogen or C₁ -C₄ -alkyl, R¹¹ is hydrogen, C₁ -C₁₂ -alkyl, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈ -cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted by one or two C₁ -C₄ -alkyl radicals, and m is from 0 to
 10. 5. The process of claim 1, wherein said thiouronium salt has formula Ib ##STR18## where R¹ to R⁴, n and X.sup.⊖ are each as defined above, R⁸ to R¹⁰ are each hydrogen or C₁ -C₄ -alkyl, R¹¹ is hydrogen, C₁ -C₁₂ -alkyl, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈ -cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted by one or two C₁ -C₄ -alkyl radicals, and m is from 0 to
 10. 6. The process of claim 1, wherein said thiouronium salt has formula IC ##STR19## where R¹ to R⁴, n and X.sup.⊖ are each as defined above; R⁸ and R⁹ are each hydrogen or C₁ -C₄ -alkyl, R¹¹ is hydrogen, C₁ -C₁₂ -alkyl, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈ -cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted by one or two C₁ -C₄ -alkyl radicals; andZ is a group of the formula CR¹² ═CR¹³ -- or --C.tbd.C-- where R¹² and R¹³ are each hydrogen or C₁ -C₄ -alkyl.
 7. The process of claim 1, wherein said thiouronium salt has formula Id ##STR20## where R¹ to R⁴, n and X.sup.⊖ are each as defined above, R⁸ to R¹⁰ are each hydrogen or C₁ -C₄ -alkyl, R¹¹ is hydrogen, C₁ -C₁₂ -alkyl, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈ -cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted by one or two C₁ -C₄ -alkyl radicals, and m is from 0 to
 10. 8. The process of claim 1, wherein said thiouronium salt has formula Ie ##STR21## where R¹ to R⁴, R⁶, R⁷, n and X.sup.⊖ are each as defined above, R⁸ and R⁹ are each hydrogen or C₁ -C₄ -alkyl, and m is from 0 to
 10. 9. The process of claim 1, wherein said thiouronium salt has formula If ##STR22## where R¹ to R⁴, n and X.sup.⊖ are each as defined above, R⁸ to R¹⁰ are each hydrogen or C₁ -C₄ -alkyl, R¹¹ is hydrogen, C₁ -C₁₂ -alkyl, C₂ -C₁₂ -alkenyl, C₂ -C₁₂ -alkynyl, C₅ -C₈ -cycloalkyl, C₇ -C₁₂ -phenylalkyl or phenyl which may be substituted by one or two C₁ -C₄ -alkyl radicals, and m is from 0 to
 10. 10. The process of claim 1, wherein said thiouronium salt has formula Ig ##STR23## where R¹ to R⁴, n and X.sup.⊖ are eachas defined above, R⁸ to R¹⁰ are each hydrogen or C₁ -C₄ -alkyl, Z is a group of the formula CR¹² =CR¹³ -- or --C{C-- where R¹² and R¹³ are each hydrogen or C₁ -C₄ -alkyl, and m is from 0 to
 10. 11. The process of claim 1, further comprising at least one further brightener. 